1. Field of the Disclosure
This specification relates to an in-plane switching (IPS) mode liquid crystal display (LCD) device, and more particularly, an IPS-mode LCD device and display device having in-cell retarder for compensating a light leakage in a front direction or a diagonal direction of the liquid crystal display panel.
2. Background of the Disclosure
As the interest in information displays and demands on the use of portable information media increase, researches and commercialization are focusing mainly on display devices which are light in weight and thin in thickness. Specifically, among such display devices, a liquid crystal display (LCD) device is a device for displaying an image using optical anisotropy of liquid crystals, and is widely applied to notebook computers or desktop monitors in view of its high resolution, high color rendering property and high image quality.
The LCD device roughly includes a color filter substrate as a first substrate, an array substrate as a second substrate, and a liquid crystal display panel between the color filter substrate and the array substrate.
Here, the color filter substrate includes a color filter having a plurality of sub color filters for rendering red (R), green (G) and blue (B) colors, a black matrix partitioning the sub color filters and blocking light transmitted through the liquid crystal layer, and a transparent common electrode applying a voltage to the liquid crystal layer.
The array substrate includes a plurality of gate lines and data lines that are arranged vertically and horizontally to define a plurality of pixel regions, thin film transistor (TFTs) as switching elements each formed at an intersection between the gate line and the data line, and pixel electrodes respectively provided on the pixel regions.
The thusly-configured color filter substrate and array substrate are assembled to each other in a facing manner, by use of a sealant which is provided along an outer edge of an image display region, thereby configuring the liquid crystal display panel.
Here, the assembling between the color filter substrate and the array substrate are achieved by a assembling key which is formed on the color filter substrate or the array substrate.
The aforementioned LCD device refers to a twisted nematic (TN) LCD device in which nematic liquid crystal molecules are driven to be perpendicular to a substrate. The TN mode LCD device has a disadvantage of a narrow viewing angle of about 90°, which results from a refractive anisotropy of the liquid crystal molecules, namely, results from that the liquid crystal molecules aligned in parallel to the substrate are aligned almost perpendicular to the substrate when a voltage is applied to a liquid crystal display panel.
Accordingly, an in-plane switching (IPS) mode LCD device with an improved viewing angle of 170° by driving liquid crystal molecules in a direction horizontal to a substrate. Hereinafter, the IPS-mode LCD device will be described in detail with reference to the accompanying drawings.
FIG. 1 is a planar view schematically illustrating a part of an array substrate of a general IPS-mode LCD device.
An LCD device actually has M×N pixels at intersections between N gate lines and M data lines. For the sake of brief explanation, one pixel will be exemplarily illustrated.
FIG. 2 is a sectional view of the array substrate illustrated in FIG. 1, taken along the line I-I′. Here, FIG. 2 illustrates a color filter substrate assembled the array substrate of FIG. 1.
As illustrated in FIGS. 1 and 2, a transparent array substrate 10 is provided with gate lines 16 and data lines 17 which are arranged in vertical and horizontal directions so as to define pixel regions. A thin film transistor (TFT) T as a switching element is provided on an intersection between the gate line 16 and the data line 17.
Here, the TFT T includes a gate electrode 21 connected to the gate line 16, a source electrode 22 connected to the data line 17, and a drain electrode 23 connected to a pixel electrode 18 through a pixel electrode line 181. Also, the TFT T further includes a first insulating layer 15a for insulation between the gate electrode 21 and the source and drain electrodes 22 and 23, and an active pattern 24 to form a conductive channel between the source electrode 22 and the drain electrode 23 in response to a gate voltage being supplied to the gate electrode 21.
For reference, a reference numeral 25 denotes an ohmic-contact layer to form an ohmic-contact between a source/drain region of the active pattern 24 and the source and drain electrodes 22 and 23.
Here, a common line 81 and a storage electrode 18s are arranged in parallel to the gate line 16 within the pixel region. Also, a plurality of common electrodes 8 and pixel electrodes 18, which generate a horizontal electric field 90 within the pixel region so as to switch liquid crystal molecules (not illustrated), are arranged in parallel to the data line 17.
Here, the storage electrode 18s overlaps a part of the common line 81 under the storage electrode 18s with the first insulating layer 15a interposed therebetween, so as to form a storage capacitor Cst.
The transparent color filter substrate 5 includes a black matrix 6 to prevent a light leakage at a region of the TFT T and between the gate line 16 and the data line 17, and a color filter 7 to render red, green and blue colors.
Alignment layers (not illustrated) which decide an initial alignment direction of the liquid crystal molecules are formed respectively on surfaces of the array substrate 10 and the color filter substrate 5 which face each other. Polarizers (or polarizing plates) (not illustrated) are arranged on outer surfaces of the array substrate 10 and the color filter substrate 5, respectively, in a manner that light transmission axes thereof are perpendicular to each other.
The general in-plane switching (IPS) mode LCD device has an advantage of having an improved viewing angle in that the common electrode 8 and the pixel electrode 18 are arranged on the same array substrate 10 to generate a horizontal electric field 90 which is parallel to the array substrate 10, and the liquid crystal molecules are aligned to be parallel to the horizontal electric field 90.
However, there is a light leakage in a diagonal direction in the general IPS-mode LCD device when a black state is formed, and thereby a contrast ratio is lowered.
FIGS. 3A and 3B are exemplary views illustrating brightness and viewing angle characteristics in a black state, in a general IPS-mode LCD device.
Here, FIG. 3A illustrates simulation results of the brightness and viewing angle characteristics in the black state, and FIG. 3B illustrates a measurement result of the brightness and viewing angle characteristics in the black state.
FIGS. 3A and 3B exemplarily illustrate the brightness and viewing angle characteristics in the black state when a 0-RT (triacetylcellulose with thickness-direction retardation value Rth close to 0 nm) film is applied between a polyvinyl alcohol (PVA) layer of the polarizer and a liquid crystal layer.
A light absorption axis of a lower polarizer is aligned to be orthogonal to a light absorption axis of an upper polarizer, and an optical axis of the liquid crystal layer is parallel to the light absorption axis of the lower polarizer.
As illustrated in FIGS. 3A and 3B, there are much light leakages at angles of 45°, 135°, 225° and 315°, which correspond to diagonal directions of the liquid crystal display panel, in the black state, and thereby brightness increases. Accordingly, the contrast ratio of the IPS-mode LCD device is lowered.
However, this problem does not result from the IPS-mode LCD device but results generally from using polarizers. That is, an IPS mode, like the IPS-mode LCD device, can decide an initial alignment state of liquid crystals such that a polarized state of light cannot be affected by the liquid crystals in all directions. In this instance, the light leakage results from the polarizers.
FIG. 4A schematically illustrates light transmission axes of the upper and lower polarizers, which are orthogonal to each other, when viewed from a front direction.
FIG. 4B schematically illustrates light transmission axes of the upper and lower polarizers, which are orthogonal to each other, when viewed in the diagonal direction.
Here, a solid line illustrated in FIGS. 4A and 4B indicates a direction of the light absorption axis of the upper polarizer, and a dashed line indicates a direction of the light absorption axis of the lower polarizer.
Referring to FIGS. 4A and 4B, even if the light absorption axes of the polarizers are orthogonal to each other, the orthogonality of the two polarizers is broken according to a viewing angle. That is, as illustrated in FIG. 4A, when the liquid crystal display panel is viewed from the front direction, the light absorption axes of the upper and lower polarizers form an angle of 90° so as to implement a black state.
However, as illustrated in FIG. 4B, when the liquid crystal display panel is viewed in the diagonal direction, the light absorption axes of the upper and lower polarizers form an angle over 90°. Accordingly, the orthogonality of the two polarizers is broken and thereby the light leakage is caused.
As such, the IPS-mode LCD device employs the way of applying the horizontal electric field to the liquid crystal layer. Accordingly, the IPS-mode LCD device exhibits the less change in phase retardation of the liquid crystals according to a voltage, and an excellent viewing angle resulting from that optical axes of the upper and lower polarizers remain perpendicular to each other in horizontal and vertical directions. However, the light leakage is caused in the diagonal direction in which the orthogonality of the two polarizers is broken and thereby image quality is lowered.
Meanwhile, even when the liquid crystal display panel is viewed from the front direction, the light leakage may be caused due to external stress applied to the liquid crystal display panel because a glass substrate obtains refractive anisotropy due to interference between a structure supporting the liquid crystal display panel and the liquid crystal display panel and stress applied thereto during an array process and a color filter process.
Glass having ideal anisotropy generates a retardation value (Re=β×t×F) in proportion to stress F applied during a fabricating process, and accordingly obtains refractive anisotropy. Here, β and t denote a photoelastic coefficient and thickness of the glass, respectively.